Apparatus for densification of porous billets

ABSTRACT

A porous preform is densified by heating while emersed in a precursor liquid. Heating is achieved by passing a current through the preform or by an induction coil immersed in the liquid. Ways to control the densification process are also described.

This invention relates generally to the densification of porous billets and more particularly to the heating of porous billets while immersed in a liquid used for densification.

Densification of porous billets is a technique used to make composite materials. One particularly useful type of porous billet is a fibrous preform as conventionally used to make fiber reinforced composites. Such things as nozzles for rockets and high performance brake pads for aircraft or racing car brakes have been made by densifying preforms. The fibers in these cases are carbon and the material used to densify the preform is also carbon.

The fibrous preforms consist of reinforcing fibers worked into a desirable shape. Many techniques are known for making preforms. The fibers might, for example, be woven or braided into a desirable shape. Alternatively, the fibers might be formed into bats which are needled into a desired shape. In other techniques, the fibers are held together by a sticky substance such as resin or pitch which is cured or stabilized with a reactive gas. The cured matrix is then carbonized by heating to a temperature in excess of the decomposition temperature of the resin.

Regardless of how the preform is made, traditional densification processes generally involve repeated cycles of filling the pores in the preform with precursor material, curing and then heating the preform until the precursor pyrolyzes to carbon or other desirable material. Another widely used densification technique is to place the preform in a precursor gas and then heat the preform to pyrolyze the precursor gas as it comes in contact with the preform.

One drawback of these techniques is that they require a large amount of time, sometimes on the order of hundreds of hours. The densification must be performed slowly so that the pores on the outside of the preform do not get filled before the pores on the inside of the preform. Were the pores on the outside of the preform to be blocked before densification of the interior portions, insufficient precursor would reach the inner portions of the preform and it would not be fully densified.

One approach to avoid this problem is revealed in U.S. Pat. No. 4,472,454 issued Sep. 18, 1984 to Houdayer et al. (which is hereby incorporated by reference). In that patent, the preform is placed in a reaction vessel and covered with a precursor liquid. A coil outside of the reaction vessel is used to inductively heat the preform. The preform is heated hot enough to boil the liquid precursor and pyrolyze the vapor generated when the liquid boils. We theorize that the liquid cools the preform at its exterior, thereby creating a thermal gradient through the thickness of the preform. The interior of the preform, since it is not cooled by the boiling liquid, is hot enough to pyrolyze the vapor. In this way densification occurs preferentially at the interior of the preform. The overall densification occurs from the inside outward. Densification can, thus, be performed at a higher rate without concern that the pores at the exterior will be blocked and prevent densification of the preform interior.

While the densification process as described in U.S. Pat. No. 4,472,454 reduces the time needed to densify a preform, we have discovered several ways in which the process could be improved.

First, in densifying a preform, it is desirable to have the induction coil conform to the shape of the preform. It is also desirable to have the preform as close as possible to the induction coil. These requirements are important to provide uniform and efficient heating of the preform. Uniform heating is important to provide a desirable densification.

To adjust the apparatus of Houdayer et al. to meet these requirements entails reshaping the coil based on the shape of the part to be densified. It also entails reshaping the reaction vessel and coil for each part. Such a requirement is undesirable because it can be costly or time consuming. Moreover, we have discovered that cooling due to the boiling of the liquid, which is needed to create the desired temperature gradient, does not occur if the reaction vessel is too close to the preform. If walls of the reaction vessel are too close to the preform, a phenomenon called "vapor lock" can occur. In vapor lock, vapor builds up at some point between the wall of the reactor vessel and the preform, displacing the liquid. Convective heat transfer away from the preform is greatly reduced at that point, creating a hot spot and causing formation of deposits on the outside of the preform. As a result, the part is non-uniformly densified.

SUMMARY OF THE INVENTION

With the foregoing background in mind, it is an object of this invention to provide apparatus for densifying fibrous preforms.

It is also an object to provide a more efficient method of densifying fibrous preforms.

It is a further object to provide a method of densifying preforms that can readily be adapted to accommodate different shaped preforms.

The foregoing and other objects are achieved by immersing a preform in a precursor liquid and then heating the preform. According to one embodiment, the preform is heated by an induction coil immersed in the precursor liquid.

In a second embodiment, the preform is resistively heated by connecting it to electrodes. In other embodiments, multiple methods of heating the preform are employed simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the following detailed description and accompanying drawings in which

FIG. 1 is a schematic of a reactor for densifying preforms;

FIG. 2A is a sketch of a fixture used to hold a preform in the reactor of FIG. 1;

FIG. 2B is a schematic of a coil as used in the reactor of FIG. 1;

FIG. 3 is a sketch of a preform heated with an alternative coil configuration;

FIG. 4 is a sketch of a preform heated resistively;

FIG. 5 is a sketch of a preform heated both inductively and resistively;

FIG. 6 is a sketch of a preform heated with an alternative coil configuration;

FIG. 7 is a graph of densification profiles at various power levels;

FIG. 8 is a representative graph of input power to the coil in the reactor vessel versus time; and

FIG. 9 is a sketch of a preform with a solid core.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a reactor 100 suitable for performing rapid densification according to the method described in U.S. Pat. No. 4,472,454. When an induction coil, such as coil 104, is used to heat a preform, reactor 100 is preferably made from nonmagnetic materials such as quartz, glass, stainless steel, ceramic, PMC or combinations thereof.

Reactor 100 contains a cavity 102 in which a preform (not shown) is densified. In operation, cavity 102 is filled with a precursor liquid sufficient to at least cover the preform (not shown). The precursor liquid is any liquid which will boil and create a vapor containing chemicals that will deposit at a temperature to which the preform (not shown) can be heated. The precursor liquid should also be a dielectric. Preferably, the dielectric constant of the reagent liquid should be above 0.5, more preferably above 1, and most preferably above 1.5. To deposit carbon on the preform, a hydrocarbon with appropriate boiling point such as cyclohexane, n-hexane or benzene could be used. To deposit silicon carbide, methyltrichlorisilane, dimethyldichlorosilane, methyldichlorosilane or other organosilane or organosilane mixtures could be used. Also, the reagent liquid could be chosen to co-deposit materials. For example, a mixture of silicon carbide and silicon nitride can be deposited using tris-n-methyl amino silane or other silazane compound.

Within cavity 102, an induction coil 104 is positioned. In operation, induction coil 104 will be covered by the precursor liquid and operate to heat the preform (not shown). Coil 104 is made from copper or other highly conductive material which does not react with the precursor liquid even if heated.

Electricity is provided to coil 104 through busses 106. Busses 106 are made of a highly conductive material, here copper. Currents of hundreds of amperes to thousands of amperes are preferably used to provide sufficient power to heat the preform (not shown). Because of the large amount of current, busses 106 must have sufficient cross section to avoid excess heating. Busses 106 may contain water passages 105 to carry cooling water through busses 106 and through coil 104.

Busses (106) are connected to a power supply (not shown). An AC supply is used. The voltage, current, frequency and shape of coil 104 are determined by the shape and geometry of the preform as well as preform properties using known techniques used to design induction heating apparatus. Typically, the voltage will be in the range from 5 to 750 V. The frequency will be in the range of 0.1 KHz to 300 MHz.

Busses 106 pass through seal 107 to enter chamber 102. As chamber 102 contains a precursor liquid during operation, seal 107 must be resilient and also resistant to chemical attack by the precursor liquid. It should also electrically insulate busses 106 from reactor 100 in the event reactor 100 is formed from conducting components. For example, silicone rubber could be used to seal the opening in reactor 100 through which busses 106 pass.

It is a matter of convenience that busses 106 enter the lower portion of reactor 100. If busses 106 entered the upper position of reactor chamber 102, seal 107 would still be needed. It would not have to prevent the escape of liquid, but it would have to prevent the escape of vapor from chamber 102. Busses 106 could even enter chamber 102 by moving down stack 136, in which case no special seal would be needed. However, it is desirable to keep busses 106 as short as possible to reduce power loss in the busses.

Precursor liquid is supplied to reactor 100 through precursor input 108 via valve 110. Initially, chamber 102 is filled with precursor liquid at least of sufficient quantity to cover the preform (not shown). In operation, precursor liquid may be consumed in the deposition reaction or escape from reactor 100 as vapor. Accordingly, precursor input 108 may be utilized during operation of reactor 100 to replace precursor liquid which is dissipated.

During a densification operation, the liquid precursor may become clouded. Accordingly, valve 114 may be opened to allow precursor liquid to flow through reagent return 112 to filter 116 where it is filtered and pumped back into reactor 100. Filter 116 may be any suitable filter such as a porous ceramic screen or, more preferably, charcoal.

The reagent liquids as used herein are potentially inflammable. Accordingly, it is preferable that the densification operation be performed in an inert atmosphere. For example, nitrogen gas may be used. To purge cavity 102 of air, valve 120 is opened to allow nitrogen to flow through input 118. Valve 124 may be opened to more rapidly and effectively purge vapor recovery system 130. Once the atmosphere in chamber 102 is replaced by nitrogen, valve 128 may be opened to provide nitrogen directly into vent stack 136. This flow of nitrogen will prevent air from reaching cavity 102 and valves 120 and 124 may be closed. Closing valves 120 and 124 reduces the flow of gas through vapor recovery system 130. Vapor recovery system 130 may therefor operate more efficiently.

Vapor recovery system 130 is a system of the type known in the art for recovering vaporized liquids. Such a system will reduce the amount of waste generated in the process and the amount of precursor used.

In operation, a preform (not shown) is placed in cavity 102 in close proximity to coil 104. Example coil locations are shown in more detail in FIGS. 3, 5, and 6. The preform is preferably placed in a support fixture to firmly hold the preform at a constant position in relation to the reactor and coil. The exact shape of the fixture is based on the shape of the preform. Such a fixture could be supported in any convenient way, such as on lip 132.

It may be desirable to use different sizes or shapes of coils based on the shape of the preform. For this reason, coil 104 is connected to busses 106 at connector 134. Connector 134 continues the electrical circuit comprising busses 106. It also continues the water flow circuit formed by channels 105. Connector 134 may simply be a block of metal allowing anchoring points for screws (not shown) to hold the base of coil 104 to busses 106. The joints in the water flow circuit could be sealed by flexible "O" rings or in some other convenient fashion. The material must be resistant to degradation in both water and the precursor liquid. Viton or silicone rubber may be used for this purpose. Other attachment arrangements, such as slots and grooves or clips, could also be used.

FIG. 2A shows a sketch of a mounting fixture 200 for use in conjunction with the reactor of FIG. 1. Fixture 200 contains a ring 202 of appropriate size to seat on lip 132 (FIG. 1). Screws 203 pass through ring 202. Screws 203 may be screwed into lip 132 (FIG. 1) to attach fixture 200 to reactor 100 (FIG. 1). Alternatively, screws 203 may just rest on lip 132 (FIG. 1). In this way, screws 203 act to adjust the vertical angle of fixture 200 relative to lip 132 (FIG. 1).

Being able to adjust the vertical angle of fixture 200 can be useful if coil 104 is fixedly attached to reactor 100 (FIG. 1). As fixture 200 holds a preform, adjusting the vertical angle of fixture 200 will also adjust the position of the preform relative to coil 104. As it is preferable that the preform be positioned so as to be concentric with the coil, being able to adjust either the location of the preform or the coil is desirable.

Fixture 200 comprises vertical members 204A and 204B. A horizontal member 206 spans vertical members 204A and 204B. A post 208 is attached to horizontal member 206. A preform is attached to post 208 in any convenient way. For example, if the preform is woven around a mandrel, a pin through the mandrel might be inserted into post 108. As horizontal members 204A and 204B, vertical member 206 and post 208 will be in close proximity to induction coil 104, it is preferable that they be made of nonmagnetic material (having a magnetic permeability of approximately 1). They should, however, be of a material strong enough to provide support to the preform. Glass epoxy composite could be used. As post 208 may be in contact with a preform which is heated, it should be made of a material which is a good thermal insulator and can withstand high temperatures. Post 208 is more preferably made from quartz.

To ensure the proper relationship between a preform and coil 104, it may be desirable to secure coil 104 directly to fixture 200. Coil 104 could then be secured to vertical member 204 with non-conducting pins 210.

FIG. 2B shows coil 104 in greater detail. Optionally, coil 104 is made of a plurality of coil segments 251, 252, 253, and 254 electrically connected in parallel. Coil segments 251-254 are connected to conducting rods 260A and 260B. Busses 106, which supply power to coil 104 are connected to conducting rods 260A and 260B. As shown in FIG. 2B, busses 106 are connected to the center of conducting rods 260A and 260B. Coil segments 251-254 are thus symmetrically disposed around the source of power. As a result, any voltage drops, which may be significant when large currents are used, are averaged out along the length of coil 104. More uniform heating of a preform thus results.

To provide more uniform heating, the length of each coil segment may optionally be different. For example, since coil segments 252 and 253 are closer to the power feed from busses 106, they could be made longer, such as by having more turns, than coil segments 251 and 254. Coil segments 252 and 253 could be sized such that the resistance in the circuit from busses 106 through any coil segment 251-254 is identical.

Coil 104 may optionally be designed to have non-uniform turn density along its length to account for gravity effects on the process. For example, vapor generated by the boiling precursor liquid will rise along the preform. The amount as well as the velocity of the vapor will be greater at the top of the preform than at the bottom. As a result, heat transfer out of the preform may be different at the top than at the bottom. To counter this effect, it is possible to structure coil 104 to provide different heating at the top of the preform. For example, the turn density of the coil could be less at the top or the spacing between the coil and the preform could be less at the top.

EXAMPLE 1

A tubular preform of 1.5" inside diameter and wall thickness of 38 to 40 mils and 6 inches long was built up from a plurality of overlapping sheets of conventional carbon/phenolic material in a format called involute wrap. The preform was carbonized by heating in excess of 650° C. Initial bulk density of the preform was 1.3 g/cc. The preform was densified in a reactor as shown in FIG. 1 with cyclohexane as the precursor liquid. A graphite core was placed in the center of the tube to act as a susceptor. The power supply provided 30 kWatts at a frequency of 160 kHz. Preform temperatures between 900° and 1500° C. were achieved. After 4 hours, the bulk density was 1.83 g/cc and apparent density as determined by mercury porosimetry was 2.01 g/cc. The porosity of the part was very low, 6.2%. Testing showed that the tube had a compressive strength of 26.3 ksi and a modulus of 44.1 msi.

FIG. 3 shows the relationship of a preform 302 to a coil 304 inside a reactor 300. Preform 302 is in the center of coil 304 and both are immersed in a precursor liquid 308.

For induction heating applications, the coil is generally shaped to conform to the part to be heated. The diameter of the coil might be smaller where the diameter of the part is smaller. Alternatively, the turn density of the coil can be increased in areas where the object to be heated is further from the coil.

Coil 304 could be shaped to conform to preform 302. Alternatively, a second coil, coil 306, may be inserted inside preform 302. The current flow through each coil should be such that the magnetic flux generated by each coil are in phase in preform 302. Preferably, both coil 304 and 306 are connected to the same power supply to ensure that the currents through them are in phase. The arrangement of FIG. 3 is, of course, only useful for hollow preforms. With both coils 304 and 306 in place, the magnetic flux is more uniform throughout preform 302. Regions of preform 302 which are further from coil 304 and thus not effectively heated by coil 304 are closer to coil 306 and more effectively heated by it. In this way, uniform preform heating may be achieved without specifically engineering a coil for each preform.

FIG. 4 shows an alternative method of heating preform 402. Here, preform 402 is in the shape of a bar or a rod. Preform 402 is clamped between electrodes 404. Preform 402 is clamped in any convenient means to provide good electrical and mechanical connections.

Electrodes 404 are made from any convenient material which can carry the current required to densify the part without heating significantly or reacting with the precursor liquid.

Here, electrodes 404 are made from three quarter inch copper rods which have slits 406 formed in them. Copper shims 408 are placed in the slot around preform 402. The shims 408 are pressed into preform 402 by bolts 410 tightened onto threaded ends 412 of electrodes 404.

In operation, preform 402 is placed in a precursor liquid 414. Electrodes 404 are connected to a power supply (not shown) which provides a current flow through preform 402 which heats preform 402. When the resistance of the preform is low, such as when the preform is made of carbon fiber or is made of some fiber held together by a carbonized resin, a high current supply is preferable. The amount of current needed will depend on the cross sectional area of preform 402 as well as its resistivity. However, the current should be sufficient to heat some part of preform 402 above the pyrolysis temperature of precursor liquid 414. Currents on the order of 1,000 amperes are likely to be needed, though the exact current level may be set empirically based on temperature measurements of the preform. Direct current is preferable, but AC current might also be used. It may also be necessary to change the current as the densification of preform 402 proceeds. As preform 402 becomes more dense, its resistance is likely to decrease, requiring an increase in current to maintain the same level of heating. For that reason, it may be preferable to use a pyrometer to continuously or periodically measure the temperature of the preform and then adjust either the voltage of the supply to keep the desired current or to adjust the current directly.

The apparatus of FIG. 4 is particularly suitable for densifying preforms in shapes with uniform cross sections, such as bars or rods or tubes with uniform diameter and wall thickness, flat plates, or other shapes with a uniform cross sectional area perpendicular to the direction of current flow. Such preforms may be in the finished, or "net" shape. Alternatively, disks or other shapes may be cut out of the rod, bar, flat plates or other shapes after densification. In this way, parts with non-uniform cross sections may be densified. Alternatively, several parts may be cut out of one densified piece, effectively allowing several parts to be densified simultaneously in one reactor with only one power supply.

FIG. 4 shows preform 402 oriented horizontally in a reactor vessel. Any orientation could be used.

FIG. 5 shows an alternative configuration for densifying a preform 502. Preform 502 is placed inside an induction coil 504. The ends of preform 502 are coupled to electrodes 506. When coupled to appropriate power supplies as described above, coil 504 provides induction heating of preform 502 and electrodes 506 facilitate resistive heating.

Preform 502 contains a concave region 508. For uniform heating using resistive heating, preforms with concave regions or regions of nonuniform cross section are generally undesirable as the current density and hence the heating, increases in concave regions. Likewise, concave regions are detrimental to uniform heating using inductive heating since, without a special coil design, concave regions are not heated as well as surrounding areas. However, using both resistive and inductive heating tends to provide more uniform heating since hot spots associated with resistive heating tend to be canceled by cold spots of inductive heating. Accordingly, it is possible to uniformly heat preform 502 without a special coil design to conform to the contours of preform 502.

The combination of resistive and inductive heating also provides an advantage in allowing better control of preform heating. For the preform to densify most fully, it is desirable that the center of the preform initially be heated above the pyrolysis temperature of the precursor liquid. A temperature gradient is established decreasing from the center of the preform to the periphery due to the cooling effect of the precursor liquid. With this temperature distribution, deposition of densifying material preferentially occurs at the center of the preform. As densification proceeds, it is desirable that regions of the preform moving successively radially outward from the center exceed the pyrolysis temperature. Resistive heating generates heat nearly uniformly across the cross section with only second order differences due to the change in resistivity due to the temperature gradient. With the precursor liquid cooling the exterior of the preform, the resulting temperature profile is hottest in the center and coolest at the edge. Additionally, for some preforms, the resistance will drop with increased temperature such that current, and hence additional heat generation, will be concentrated at the hotter interior portions of the preform. This temperature distribution is well suited for the start of the densification cycle.

Conversely, induction coil 504 causes heat to be generated in greater amounts near the periphery of the preform. The amount of heat generated drops to 14% of its maximum value at the skin depth. The skin depth is in turn a function of frequency, decreasing in inverse proportion to the square root of the frequency. By appropriately selecting frequency using known techniques familiar to those skilled in the art of induction heating, induction coil 504 may provide greater heating at the periphery of the preform. This heat distribution is desirable at the end of the densification cycle. Accordingly, desirable results can be achieved by initially heating preform 502 resistively and then increasing the current through coil 504. Current through electrodes 506 could be simultaneously decreased if desired.

A similar heat distribution during the densification cycle could also be obtained by an induction coil above. The frequency of the power supply is initially set to provide a skin depth of roughly one quarter to one third the diameter of the preform. This skin depth, taking into account heat transfer out of the preform, will provide maximum heat accumulation at the center of the preform. Ideally, the power in the coil will be set to heat the preform to slightly above the pyrolysis temperature of the precursor liquid while the precursor liquid cools the remainder of the preform to be below the pyrolysis temperature. As the center of the preform densifies, the frequency of the power supply can be increased to heat the preform to slightly above the pyrolysis temperature in regions slightly displaced from the center. Also, the power supplied in the coil may be slightly reduced to ensure that the remainder of the preform remains below the pyrolysis temperature of the precursor liquid. Adjustments can be made in this fashion until the preform is fully densified. The exact rate of change of frequency and power will depend on the shape and composition of the preform and may need to be determined empirically.

Even if an induction coil is used without a resistive source, a desirable result may be obtained by increasing the power to the induction coil as the densification cycle proceeds.

FIG. 7 shows three curves useful in understanding how increasing power during the densification cycle can provide more complete densification. FIG. 7 shows curves 702, 704, and 706, each of which shows the density of a densified preform as a function of distance from the center of the preform thickness. Curve 702 is made at a relatively low input power. The density is a maximum at the centerline. This densification pattern results because the center heats to a temperature sufficient to cause pyrolysis of the precursor liquid. The outside of the preform is cooled such that no pyrolysis and associated deposition reactions result. Curve 706 is made at a relatively high input power. Maximum density occurs at the periphery because the periphery heats to a temperature high enough to cause pyrolysis of the precursor. Pyrolysis causes deposits which block the infiltration of the precursors into the center of the preform. Curve 704 is made at an intermediate power and shows a maximum density intermediate the centerline and the periphery.

To provide improved densification, it is desirable to initially use a power P_(o), such as was used to create curve 702. At the end of the densification cycle, it is preferable to use a power, P_(f), such as was used to create curve 706. In between, it is better to use a power which gives a curve such as 704. FIG. 8 shows a curve of desirable levels of input power as a function of time, P(t), during the densification cycle. The power is initially set at P_(o). At the end of the densification cycle at time T_(f), the power equals P_(f). During the densification cycle, the power is increased. As shown in FIG. 8, the power is increased in proportion to time, t, raised to the power n. This relationship is desirable because the chemical reaction which causes deposits increases with increasing temperature. Thus, less time is needed to densify the outer portions of preform. Also, the other portions of the preform densify to some extent while the interior is being densified. As a result, the peripheral portions of the preform densify much faster than the interior and less time is needed to densify the outer portions of the preform.

The values of ∝, P_(o), P_(f), T_(f), and n depend on such things as preform size and geometry as well as on the specific precursor liquid used. It is possible to theoretically calculate those values. However, due to the complicated nature of the phenomena involved, it can be preferable to empirically determine appropriate values. Several trial runs, stopped periodically to observe the preform and measure its density, may be necessary to determine appropriate values. Values of n in the range of 1 to 3 have been observed to be satisfactory for preforms as shown herein. However, higher values may be desirable for other geometrics.

It should be noted that FIG. 8 shows a continuous change in power. The power may, however, be increased in discrete steps. Also, the same pattern of varying input power may be used regardless of the method of heating the preform. Further, the curve of FIG. 8 shows that the power is continually applied. As described hereafter, it may be beneficial to periodically pulse the applied power between an "on" state and "off" state or a reduced level. If a pulsed power source is used, the curve of FIG. 8 represents the power in the "on" state.

A further way of controlling the deposition is to adjust the pressure in the reactor chamber. Initially, it is desirable to cool the periphery of the preform so that deposition occurs principally at the interior. Cooling occurs by boiling or vaporization and convection of the precursor liquid, in addition to radiative heat transfer. Once the center of the preform has been densified, less cooling of the periphery is desirable so that densification of the exterior of the preform will densify rapidly. To reduce the cooling, the pressure of the reactor chamber may be altered. For example, the pressure might be simply increased by choking off vent stack 136.

As described above, it is desirable to have densification occur preferentially at the interior of the preform. The foregoing process control techniques relate to controlling a densification process by controlling heating of the preform. It is also possible to control the diffusion of vapor into the preform.

If more vapor reaches the interior portions of the preform or the concentrations of vapor in the interior of the preform increases, densification will occur preferentially at the interior of the preform.

One way to increase the concentration of materials which will form a deposit in the interior of the preform is to pulse the heating of the preform. Pulsing the heating allows by-products generated when the vapor forms a deposit to diffuse out of the preform when no heating is occurring or heating is reduced. For example, if cyclohexane is used as a precursor liquid, H₂ is generated as a by-product. If the power to the coil (when induction heating is used) or the power to the preform (when resistive heating is used) is periodically interrupted for a period long enough to allow H₂ to diffuse out of the preform, when heating is resumed, more cyclohexane vapor can diffuse into the preform. The concentration of the cyclohexane vapor will then be higher because the H₂ has dissipated.

The heating needs to be interrupted for a relatively short period of time. The length of time depends on the size of the preform and also the stage of densification. It might take longer for the by-products to diffuse out of the center of a thick preform then out of the edges. As a result, it may be desirable to interrupt the heating for longer periods of time during the early parts of the densification cycle when the interior of the preform is densifying. Heating, preferably, should be interrupted for a period of between 0.01 seconds and 10 minutes, more preferably 0.01 to 3 minutes. Heating should be interrupted at intervals of a length inversely proportional to the rate at which the by-products are generated, preferably between about 0.01 seconds and 3 minutes.

Interrupting the heating also provides an additional advantage of making a stronger finished part. The strength of the densified part is due in part to the strength of the deposited material. The strength of the deposited material is in turn dictated by its microstructure. As the material is deposited, crystalline domains grow. The part is stronger though, if all the domains are small. Interrupting heating long enough for the part to cool to a temperature which would cause renucleation, results in smaller domains. Domains smaller than the diameter of the fiber used to make the preform, typically below 5 microns, are considered small.

The heating should be applied to the perform for the time it takes the domain to grow to the desired size. Times of approximately 0.1 seconds to 5 minutes are typical. The heating should than be interrupted for long enough for the preform to cool below the renucleation temperature. Times of 0.01 seconds to 10 seconds are typical. Since the deposition is exponentially dependant on temperature, cooling of as little as 10° to 200° C. may be sufficient to cause renucleation.

Controlling the grain size is also important in making friction material, such as is used in brakes. Smaller domains may lead to a different coefficient of friction than larger domains. Controlling the domain size thus allows the materials within the desired coefficient of friction to be made.

It should be noted that the pulses need not be of constant duty cycle or occur at constant intervals. For example, as shown in FIG. 8, the power level may change during the densification cycle. The pulse characteristics might be changed with the power level.

An alternative way to increase the deposition of material in the interior of the preform is through the use of pressure waves in the liquid precursor. These waves are accentuated in the vapor as density waves which force precursor into the part and draw by-products out. In the system as described above, pressure waves exist in the liquid due to generation and contraction of bubbles associated with boiling the liquid precursor. The magnitude of the waves could be increased by cooling either the precursor liquid or the exterior of the reaction vessel. For example, in FIG. 1 filter 116 could contain a refrigeration section to cool the liquid. Alternatively, reactor 100 could be jacketed by water or some other cooling mechanism.

An alternative approach to generating pressure waves in the precursor liquid is to place one or more transducers in the precursor liquid. An acoustic or ultrasonic transducer could be used. The transducer could be pulsed to generate waves in the precursor liquid. Mechanical agitation or stirring of either the preform or precursor liquid could also be used.

FIG. 6 shows an alternative coil arrangement. Here, a disk shaped preform 602 is placed between "pancake" coils 604. Pancake coils 604 will provide a more effective heating in the center of some preforms, such as preform 602, than would a coil shaped, for example, as coil 304 (FIG. 3). Pancake coils are also useful for preforms in which the through-the-thickness resistivity is high or for heating preforms which contain edges which would be along the axis of a coil such as coil 104 (FIG. 1).

FIG. 6 shows pancake coils 604 to have uniform spacing between successive turns. Nonuniform spacing may be desirable in some instances. For example, if preform 602 is a disk with a hole in its center, the turn density might be increased in the region halfway between the outside of the preform and the outside of the hole in the center of the preform. Design techniques conventionally used for induction heating systems are preferably employed.

FIG. 6 shows preform 602 resting on an open mesh 610. Open mesh 610 or other similar support structure holds preform 602 while still allowing precursor liquid 608 to reach the under surface preform 602. Generally, preforms for carbon-carbon composites are made of fibers held together by resin or pitch. The resin or pitch is then heated to a high temperature, converting it to carbon. The carbon is still porous and must still be densified. However, the preform is generally fairly rigid and many ways of supporting the preform are possible.

The preform of FIG. 6 is called a dry preform because it is not held together by carbonized resin or pitch. One type of dry preform is a "needled" preform, which is made by stacking up layers of fibers and poking barbed needles through the stack. The needles drag fibers through the layers locking the layers together. The resulting preform is less rigid and may need to be supported throughout its length on a structure such as a frame or mesh 610.

Dry preforms also have a higher resistivity than a preform held together with resin or pitch, thus, a higher frequency to is required heat these materials efficiently. As the preform begins to densify, the frequency may need to be decreased to compensate for the decrease in resistivity. Similar adjustments may be needed for resistive heating. The current may need to be increased to compensate for the decreased resistance.

For some preforms, very high frequencies will be needed to provide effective induction heating. Resistive heating as described herein could be used. Alternatively, high frequency energy can be generated from a microwave source rather than an induction coil. If microwaves are to be used, the reactor vessel should be made from a material which reflects microwave energy and shaped like a cavity used in a microwave furnace. Where necessary, openings in the reactor vessel must be smaller than one quarter wave length of the frequency used or covered with a conductive mesh having openings less than one quarter wavelength. Frequencies in the range of 300 MHz to 300 GHz could be used, more preferably, frequencies in a range of 915 MHz to 2.45 GHz could be used.

An alternative to using such a high frequency power source is to incorporate a susceptor into the preform. A susceptor is a material which will heat readily. Heretofore, graphite pieces have been used as susceptors. FIG. 9 shows in cross section a disk shaped preform 900. Preform 900 has a core 904 which acts as a susceptor. Core 904 may be graphite or may be carbon or some other material which readily heat when exposed to energy of the type generated by the heating source in use.

The porous portion 902 of preform 900 surrounds core 904. Portion 902 may be applied by molding conventional carbon/phenolic material around core 904. As an alternative, a sheet of carbon felt might be placed above and below core 904. The sheets of carbon felt might then be needled together.

Core 904 might be any susceptor material. A particular useful susceptor might be a previously densified disk. For example, aircraft brakes are made from carbon/carbon disks. In use, the disks wear down and are usually discarded. If those worn disks were machined down, they could be used to form core 904 to make new disks. In this fashion, used disks are actually turned into new parts.

Having described various embodiments of the invention, one of skill in the art may make various alternative embodiments without departing from the invention. Limitless reactor shapes are possible. Many suitable materials could be found for making the equipment described herein. Also, porous billets in the form of fibrous preforms have been described, but many types of preforms could be densified with the method and apparatus defined herein. Further, carbon fiber preforms densified with carbon have been used as an example. Carbon fiber preforms could be densified with ceramics or ceramic fibers could be densified with either carbon or ceramics. Also flux concentrators could be used in conjunction with induction coils as described herein. For example, FIG. 6 shows a pancake coil, which produces a symmetrical field pattern. Flux concentrators such as ferrite balls or the commercial product Fluxtrol flux concentrator sold by the Fluxtrol Company could be used. The flux concentrator might, for example, be placed on the outside portions of the coil away from the preform to direct more of the generated flux towards the preform. Accordingly, the invention should be limited only by the spirit and scope of the appended claims. 

What is claimed is:
 1. A reactor for densifying a porous billet comprising:a) a reactor for containing a precursor liquid; b) a means, within the reactor, for heating the porous billet to a temperature sufficient to cause the precursor liquid to pyrolize and deposit material within the pores of the porous billet; and c) means coupled to the reactor, for condensing vaporized precursor liquid and returning it to the reactor.
 2. The reactor of claim 1 wherein the means for heating comprises an induction coil.
 3. The reactor of claim 2 additionally comprising a fixture for holding the porous billet and for holding the induction coil.
 4. The reactor of claim 3 additionally comprising means for adjusting the position of the fixture relative to the coil.
 5. The reactor of claim 2 wherein the induction coil is a pancake coil.
 6. The reactor of claim 5 additionally comprising a porous means for supporting a porous billet adjacent to the induction coil.
 7. The reactor of claim 2 additionally comprising a porous billet immersed in the precursor liquid, said porous billet having a nonuniform cross section and wherein the induction coil is shaped to conform to the cross section of the porous billet.
 8. The reactor of claim 2:a) whereini) the coil has a hollow section for receiving a cooling fluid and ii) wherein the reactor has at least one wall; and b) additionally comprisingi) a power bus passing through the wall of the reactor said power bus having a hollow section for carrying a cooling fluid vessel into the precursor liquid and ii) connection means for removably connecting the induction coil to power bus, said connecting means including means for electrically connecting the induction coil to the power bus and for making a fluid seal between the hollow section of the induction coil and the hollow section of the power bus.
 9. The reactor of claim 1 wherein the means for heating comprises electrodes with connectors adapted to make electrical contact to the porous billet.
 10. The reactor of claim 9 wherein the electrode comprises a conducting element having a slot therethrough adapted to receive the porous billet and a nut threaded onto the conducting element.
 11. The reactor of claim 1 wherein the means for heating comprises:a) an induction coil; and b) electrodes with connectors adapted to make electrical contact to the porous billet.
 12. The reactor of claim 1 additionally comprising means for cooling the precursor liquid.
 13. The reactor of claim 12 wherein the reactor has walls surrounding the precursor liquid, and the means for cooling the liquid comprises means for cooling the walls.
 14. The reactor of claim 1 comprising means for generating waves in the precursor liquid.
 15. The reactor of claim 14 wherein the means for generating waves comprises a transducer.
 16. The reactor of claim 1 additionally comprising means, connected to the reactor for filtering the precursor liquid.
 17. A reactor for densifying a porous billet comprising:a) a reactor for containing a precursor liquid; b) means, within the reactor, for heating the porous preform to a temperature sufficient to cause the precursor liquid to pyrolize and deposit material within the pores of the porous billet; and c) wherein said means for heating comprises an induction coil having a plurality of turns of conductive material with the spacing between turns varying along the length of the induction coil.
 18. A reactor for densifying a porous preform comprising:a) a reactor vessel for containing a precursor liquid; b) means, within the reactor, for heating the porous preform to a temperature sufficient to cause the precursor liquid to pyrolize and deposit material within the pores of the porous preform; and c) means, coupled to the reactor vessel, for purging air from the reactor vessel and blocking the passage of air into the reactor vessel during operation of the reactor.
 19. The reactor of claim 18 wherein the means for heating comprises an induction coil.
 20. The reactor of claim 18 wherein the means for heating comprises electrodes with connectors within the precursor liquid adapted to make electrical contact to the porous preform.
 21. The reactor of claim 19 additionally comprising a fixture for fixedly attaching the porous preform to the induction coil.
 22. The reactor of claim 18 additionally comprising a means, connected to the reactor vessel, for condensing vaporized precursor liquid and returning the condensed precursor liquid to the reactor vessel.
 23. The reactor of claim 18 wherein the means for purging air from the reactor comprises at least two inlets for inert gas connected to the reactor.
 24. The reactor of claim 18 additionally comprising a vapor recovery system connected to the reactor vessel and an exhaust pipe coupling the vapor recovery system to the atmosphere and wherein one of the inlets for inert gas is connected to the reactor vessel and one of the inlets for inert gas is connected to the exhaust pipe. 